1. Technical Field
Embodiments of the present invention relate generally to electrical motors and, more particularly, but not exclusively, to transverse flux machines in which the electromagnetic force vector is perpendicular to the magnetic flux lines.
2. Description of Related Art
Linear motion systems are in common use in industry, with different systems available to handle a variety of applications.
One type of system uses a ball and screw arrangement, in which the ball screw is rotated by a static rotary motor. An advantage of this arrangement is that the electric cables that drive the system are statics, and therefore may be fixed to the main body of the machine. Some of the disadvantages however include limits in speed, and relatively high vibration, friction, and acoustic noise.
Where faster speed and smooth, high precision movements are required, electric motors having a stationary element and movable element are frequently employed. In some linear motors, for example, the movable element includes a current-carrying winding wrapped around a magnetic core of magnetizable material such as iron or steel, and the stationary element contains permanent magnets.
These linear motors have a disadvantage however in that the movable winding needs to be connected by cable to the driver current of the motor. In order to avoid deterioration of the connecting cables, a cable arrangement that is costly and complicated is usually required. Further, the cable connection creates mechanical friction and perturbations that affect the smoothness of the motor movement.
An alternative type of linear electric motor reverses placement of the components, by placing the windings and magnetic core on the stator and the permanent magnets on the moving element. An example of this motor configuration is shown in U.S. patent application US2007/0114854 to Miyamoto. A problem with this configuration however is that the windings and magnetic core are extended along the full length of the linear motor, which makes the motor relatively heavy and expensive. Further, these motors have low efficiency since only the small section of the winding that is in front of the moving element is active.
Both of these common types of linear motor also have a strong attraction force between the moving and the static elements. The attraction force acts as a constraint on movement, requiring additional current input to overcome, which further reduces motor efficiency.
Rotary motors work on the same principle as the above linear motors, but produce as mechanical output a shaft that rotates rather than one that moves back and forth along a straight-line path. Some rotary motors of the AC synchronous and DC brushless type accordingly have permanent magnets on the rotor, and winding and magnetic material on the stator. However, when these motors operate at high speed, the rotating magnets on the rotor become subject to a strong centrifugal force. This creates a risk that the magnets might dislodge and fly off. In order to reduce this risk, costly design features are needed.
One approach to this problem involves inserting the permanent magnets in dedicated slots inside the rotor magnetic material. This type of motor is called an Internal Permanent Magnet (“IPM”) motor, and an example is shown in U.S. patent application 2007/0278886 to Fujiwara. A problem with IPM motors is that they include a “reluctance torque” which must be compensated by the electronic drive in order to maintain a constant torque for a given current amplitude at variable rotation angles. This design accordingly limits the maximum torque available to a lower value, and degrades the linear relationship between motor current and torque output.
Another type of rotary motor, called “Direct Drive”, is designed to have a relatively large number of poles in order to obtain high torque. These motors are commonly used to directly activate loads at a relatively low speed, without the need for reduction gear. In such motors however the winding is arranged around the poles, so that the space needed for the winding limits the number of poles for a given diameter of the motor. Accordingly, in order to obtain the desired high torque, the direct drive motor becomes relatively large and heavy.
In recent years a different type of electric motor called transverse flux machines (“TFM's”) have gained in popularity. Whereas in standard electric motors, such as those described above, the electromagnetic force vector is parallel to the magnetic flux lines, in TFM's the electromagnetic force vector is perpendicular to the magnetic flux lines. The TFM design allows the pole number to be increased without reducing the magnetomotive force per pole, and is therefore capable of producing power densities higher than a conventional machine. Some common disadvantages of TFM's include low power factor and complex construction with three-dimensional magnetic fields. The use of lamination in TFM's is complicated, so that sometimes the costly soft magnetic composite materials are required.